Oracle Enterprise Manager is Oracle's GUI-based database management tool. It is possible to manage the oracle environment using SQL and PL/SQL but OEM gives you a console based framework to help you with almost all of your day-to-day management activities, including tasks such as backup, recovery, export, import and data loading, it also makes it easier to use the oracle advisors.

There are two version of OEM - a single instance version called Database Control and a system wide version that lets you manage all your systems called Grid Control. Most dba's do not like using GUI's but it makes sense to use the GUI for the easier day-to-day administration tasks like adding users, resizing tablespaces, etc, I think that knowing both SQL and PL/SQL and the GUI will make you a better dba in the long run.

The benefits of using the OEM

Out-of-the-box Management

Installing and configuration of the OEM (both database and grid control) is a breeze, if using the DBCA to install Oracle it will automatically be installed for you, configuring the OEM is a trivial task.

Web-based Management

OEM can be configured to use both HTTP or HTTPS, so security can be tightened if required. Being web based all you need is web browser to access the OEM.

Real-Time Monitoring

Normally you have to write scripts or use a 3rd party tool to perform real-time monitoring, which can cause additional expensive or time, OEM will perform real-time monitoring right from the off. The new hang analysis feature in 10.2 helps during a database hang, when SQL scripts can't get information back to you quickly, by attaching directly to the SGA bypasses SQL and helps you out when normal SQL is used.

Complete Environment Monitoring

A poor performing database could be the result of a problem anywhere in the application stack (web server, app server, etc). OEM can monitor the performance of all the components in the stack not just the Oracle database. It can monitor

The entire Platform

End user experience

Systems and web applications availability

The extended network components

Business transactions

Historical data analysis and change tracking

Application Performance Monitoring

Oracle uses alert and notification systems to monitor the database, these alerts can identify problems quickly thus using a pro-active approach, thresholds can also be easily adjusted with the OEM. It can identify

Monitors the performance and availability of the system

Indicates outages and bottlenecks throughout the application system

Analyzes the root cause of performance with drill-downs

Minimizes application downtime through the use of efficient problem-resolution tools.

Scaleable Management Capability

OEM is highly scalable, the only requirement is that any new servers require the management agent started.

Consolidated Database Management

The home page of OEM will contain everything you need for a quick overview of your environment, this means that any problems can be identified very quick, a few more clicks and you have advanced performance charts available.